1. Field of the Invention
The invention generally relates to management of loads connected to an electrical power distribution grid, and more particularly to a controller for controlling operation of at least one electrical load operating on an AC supply provided by such a grid.
2. Description of the Prior Art
Due to the number of power failures encountered by power utilities, on the grids they use and the general increase of energy consumption causing critical peaks of electricity production, various devices or systems have evolved in the field for managing electricity distribution on grids and controlling the distribution of the power to the loads connected to these grids.
Known in the art is U.S. Pat. No. 4,471,232 (PEDDIE et al.), which describes a method and an apparatus for controlling loads on an electrical grid. The apparatus, which comprises a switching device, allows an electrical distribution center to shut off the current supplied to the loads according to an established hierarchy defined by associating a given priority level with each load, such that the less important loads are shut down before the more important ones. The apparatus controls the time period during which the loads are disconnected from the grid by means of a timer programmed to that effect. A control signal common to all of the switching devices is generated on the electrical grid and consists of a detectable interruption of the AC supply signal.
Broadcasting on grids control signals containing information in their amplitude is a risky method considering the normal perturbations to which the grids are subjected, generated for example by cutoffs of circuit breakers, terminal commutations of transformers, voltage regulators, and high voltage capacitors, as well as lightening inductions. Errors may happen due to the destruction of the information contained in a control signal, or even worst, the generation of random control signals. In all cases, the results are uncertain and the efficiency of the energy management carried out by such an apparatus is questionable. The control signals might not be well received by each apparatus because of the inherent properties of the grid such as inductance, capacitance and resistance altering the signals. Furthermore, no means has been put in place for carrying out preprogrammed actions depending on the state in which the grid operates, means that can prove useful when for example the grid is subjected to an impending power failure.
Also known in the art is U.S. Pat. No. 4,847,781 (BROWN III et al.), which proposes a system for energy management that can be integrated through an electrical distribution or distribution grid. The devices for controlling electric appliances are intended to receive a common control signal having a high frequency in the range of audio signals, superimposed to the AC supply normally provided by the grid.
Also known in the art is U.S. Pat. No. 4,359,644 (FOORD), which proposes a device for controlling consumer loads, especially when a need to shed loads from the grid under predetermined conditions is expected. Broadcasting of control signals is made through an energy distribution line by magnitude distortion of the AC supply signal, the allowed distortion being up to 0,4% of its power.
Also known in the art, are U.S. Pat. Nos. 4,130,874 (PAI), 4,215,394 (GALLOWAY et al.) and 4,419,665 (GURR al.), which propose various devices for management of grids, applying methods more or less complicated from one another for broadcasting control signals to the devices in an attempt to control operation of the loads connected to the grids.
The prior art documents teach either the amplitude modification of the waveform of the AC supply (the information is contained in the magnitude of the signal) or the generation of an additional signal (such as a signal superimposed to the AC supply) to command operation of a device which in response controls the load's operation or supply. Both methods and the involved control devices lack the ability to "sense" the state in which the grid operates. Those control devices cannot alone, for example, detect a local from a general grid failure, nor can act by themselves in an attempt to avoid the failure of the grid or even minimizing its losses. They only play a passive role in the management of the grid, always waiting orders from a remote commanding device linked to them. Then, intermediary steps have to be performed to determine which commands should be transmitted to the control devices, in the case that commands can still be broadcast through the grid (unless a complete additional communication network is implemented for that purpose, such as some documents have proposed), which means that all the connections shall still be effective, that no cutoff devices or protective equipment on the grid have been actuated.